1. Field of the Invention
This invention relates to the electrodeposition of gold, and more particularly to a method for improving the current efficiency of an electroplating bath.
2. Description of the Prior Art
Gold electroplating has been used in a variety of applications ranging from purely decorative to industrial. Recently, the expanding electronics industry has created a great demand for gold electroplating processes. The chemical and physical stability, good conductivity, and corrosion resistance of gold makes it ideally suited for use in electronic devices. Gold has been used in integrated circuits for bonding pads, contacts, and current conductors. It has also been useful in making ohmic and rectifying contacts in semiconductor devices since it forms an eutectic with silicon and germanium. Other electronic applications include plating of copper headers and housings for semiconductor diodes where, for example, copper ions would contaminate the device.
Unfortunately, a major problem with gold plating is that the current efficiency (amount of gold plated per quantity of electricity applied) decreases with time. Therefore, the length of time for plating must be increased to ensure a predetermined minimum thickness. This may lead to overplating and consequent waste of gold. Furthermore, the lower plating efficiency may have an effect on the porosity and quality of the deposit. Therefore, when the current efficiency of the bath falls below a certain minimum, the bath must be replaced. Frequent replacement of the bath is wasteful not only of gold, but also of personnel time.
Accordingly, some research has been done to ascertain the cause of the drop in current efficiency with electrolysis time. Conventional plating baths typically use gold I species as the source of gold. Polarographic studies have demonstrated that gold III species accumulate in these solutions with time and have a deleterious effect on the current efficiency. The prior art has proposed the reduction of gold III by chemical reducing agents. Generally, hydrazine has been used. U.S. Pat. No. 4,067,783 suggests a treatment of 0.25 ml 85% hydrazine per 100 ml of bath solution accompanied by heating for a period of time. The problem with hydrazine is that it is a noxious, carcinogenic chemical. Furthermore, excessive hydrazine should be avoided due to its adverse effect on the chemical and physical properties of the gold plating solution. Active charcoal has also been used to restore current efficiency by absorbing certain impurities, but it is not as effective as hydrazine treatment and also absorbs gold species.
In addition to a build-up of gold III species, E. D. Winters, Plating, Vol. 72, 213 (1972) demonstrated that oxygen from the atmosphere dissolves into the bath and also decreases the current efficiency as the reduction of the oxygen competes with the reduction of gold I into elemental gold. The prior art suggests inert gas purging or chemical oxygen scavengers for removing the dissolved oxygen. In particular, nitrogen sweeping of the plating solution before and during plating has been suggested. However, this technique may not always be practical. Chemical reducing agents such as hydrazine have been employed. Addition of a complex which will yield a sulfite ion has also been suggested since sulfite reduces the dissolved oxygen to sulphate. However, sulfite can not be used where the pH of the bath is less than 7.